1. Field of the Invention
The present invention relates generally to optical phase modulators and relates specifically to optical phase modulators controlled by a pulsed optical input signal.
2. Discussion of Related Art
Optical phase modulators are commonly used devices and are well known in the art. They are useful for converting an electrical, acoustical, or optical signal to an optical signal with a specific phase value. Optical phase modulators have also been developed as optical devices for logic gates and signal receivers. A variety of optical phase modulators are used to provide digital binary phase shift keying (BPSK) modulated signals. However, a more efficient and faster phase modulator which can receive a pulsed input signal of unknown phase, such as a transmission line signal, is needed.
Electro-optic phase modulators are commonly used to phase modulate optical signals using electronic signals. An electronic signal is used to produce an optical effect, such as the Pockels effect or the Kerr effect, thereby modulating the refractive index, and thus the phase delay, of an optical material. These devices are relatively slow and usually require a large electronic signal to produce a large phase shift. As well, using this method a pulsed optical signal must be converted to an electronic signal before it can be used to phase modulate another optical signal. These devices are not well suited for phase modulating one optical signal with another pulsed optical signal, and are not as efficient as an optical pulse controlled phase modulator.
U.S. Pat. No. 5,274,651 describes a method of self-phase modulation that uses a pulsed input signal. The pulsed input signal creates a self-induced intensity-dependent refractive index change and phase modulates itself based on the intensity of the pulsed input signal. A second optical signal can also be used to modulate the refractive index of a first signal according to the intensity of both signals. However, this method does not phase modulate the input signal relative to a known phase value and, therefore, the phase of the signal is still not known after modulation.
In an alternative method, a second optical signal is used to modulate the refractive index of a first signal that has a known phase. However, the optical refractive index change is a relatively weak physical effect, usually requiring large modulating signals and materials with large nonlinear properties.
Some optical phase modulators based on signal interference are used as all-optical logic devices for optically controlled phase modulation. However, these devices are limited to signals with known relative phase values. Pulsed input signals with unknown phase values cannot be properly processed and utilized by these devices because the pulsed input signals, of unknown phase, will not interfere with the device signals, of known phase, as desired.
Other means are also used also for receiving pulsed signals with unknown phase values and converting them to other signals. A heterodyne receiver uses beat interference with a local oscillator to detect a received signal and convert the beat interference to an electronic signal. A homodyne receiver tries to determine the phase of the received signal by combining the received signal with several signals from a local oscillator having various phase values. However, heterodyne and homodyne receivers require some electronic control and are more complex, less efficient, and slower than a pulse controlled phase modulator.
A simpler, more efficient, and dedicated optical phase modulator that can convert a pulsed input signal with unknown phase to a digital BPSK signal is needed.